cmake: add code to detect system brotli library (#952)

Release/cmake/cpprest_find_brotli.cmake

@@ -3,8 +3,17 @@ function(cpprest_find_brotli)
     return()
   endif()
 
-  find_package(unofficial-brotli REQUIRED)
 
-  add_library(cpprestsdk_brotli_internal INTERFACE)
-  target_link_libraries(cpprestsdk_brotli_internal INTERFACE unofficial::brotli::brotlienc unofficial::brotli::brotlidec unofficial::brotli::brotlicommon)
+  find_package(PkgConfig)
+  pkg_check_modules(BROTLIENC libbrotlienc)
+  pkg_check_modules(BROTLIDEC libbrotlidec)
+  if(BROTLIDEC_FOUND AND BROTLIENC_FOUND)
+	  target_link_libraries(cpprest PRIVATE ${BROTLIDEC_LDFLAGS} ${BROTLIENC_LDFLAGS})
+  else(BROTLIDEC_FOUND AND BROTLIENC_FOUND)
+    find_package(unofficial-brotli REQUIRED)
+    add_library(cpprestsdk_brotli_internal INTERFACE)
+    target_link_libraries(cpprestsdk_brotli_internal INTERFACE unofficial::brotli::brotlienc unofficial::brotli::brotlidec unofficial::brotli::brotlicommon)
+    target_link_libraries(cpprest PRIVATE cpprestsdk_brotli_internal)
+  endif(BROTLIDEC_FOUND AND BROTLIENC_FOUND)
+
 endfunction()

Release/src/CMakeLists.txt

@@ -84,7 +84,6 @@ else()
     target_compile_definitions(cpprest PRIVATE -DCPPREST_EXCLUDE_BROTLI=1)
   else()
     cpprest_find_brotli()
-    target_link_libraries(cpprest PRIVATE cpprestsdk_brotli_internal)
   endif()
 endif()